The present invention generally relates to DC/AC converters and inverters, and more specifically to bidirectional DC/AC converters or inverters that can be used for managing micro grids and in the grid storage and renewable energy fields.
A typical inverter is usually wired such that there is no ‘neutral’ connection, which is generally referred to as a Delta configuration. The other common wiring configuration is known as “Wye”, in which all three phases are wired along with a neutral connection. The Delta configuration is usually done to save on the number of power switching devices that are used (a three-phase inverter can operate with six power switching devices, and this configuration is commonly referred to as a Hex-Bridge topology). In addition, typical multi-phase inverters must operate in a “balanced” mode. This means that at any one time the sum of the currents in the system must equal zero. In the case of a typical three-phase inverter, the three phase currents must instantaneously sum to zero. The result of this requirement is that the phase angle of the currents (or power factor) is usually constant among all three phases. It would be possible to make the phases individually controllable, but in this configuration only two of the phases can be controlled, and the third phase would need to be a function of the other two phases such that the sum of the currents is equal to zero.
Another limiting factor of the three-phase hex bridge-based inverter is that the real component of the power must have the same polarity. This means that the inverter is either net sourcing power to the grid, or net sinking power to the grid on all three phases.
It would be desirable to have an inverter that can individually control multiple phases of AC power and that can inject an independent amount of reactive power onto each phase. For example, one phase could be injecting lagging reactive power while other phases are injecting leading reactive power, depending on the situation. It would also be desirable to have an inverter that can independently control reactive power, where each phase can have independent control of the real power component, such that one phase can be sourcing power to the AC connection while the other phases are sinking power from the AC connection.
A converter system is disclosed with individual real and reactive power control for each phase of a poly-phase system. The converter system couples one or more DC loads and/or sources to one or more AC loads and/or sources, each of the AC loads/sources using AC power having a plurality of AC phases, and each AC phase including a current waveform having a current amplitude and a current phase angle and a voltage waveform having a voltage amplitude and a voltage phase angle. The converter system includes a bidirectional poly-phase inverter, an AC line filter, a link capacitor, at least one DC to DC converter and a controller. The bidirectional poly-phase inverter includes a plurality of single-phase inverters, where each of the single-phase inverters has an AC side and a DC side, and handles a separate AC phase of the AC loads/sources. The AC line filter has an AC inverter side coupled to the AC sides of the plurality of single-phase inverters, and an AC source/load side coupled to the AC loads/sources. The link capacitor is connected in parallel with the DC sides of the plurality of single-phase inverters. The DC to DC converter(s) is connected in parallel with the link capacitor, and is coupled to the DC loads/sources. The controller controls the plurality of bidirectional single-phase inverters and the at least one bidirectional DC to DC converters. The controller controls the current amplitude of the current waveform of each AC phase independently of the current amplitude of the current waveform of the other AC phases. The controller also controls the difference between the current and voltage phase angles of each AC phase independently of the difference between the current and voltage phase angles of the other AC phases.
The plurality of single-phase inverters can consist of only three single-phase inverters; and all of the AC loads/sources can use three phase AC power. Each of the single-phase inverters can be a bi-directional single-phase DC/AC inverter galvanically isolated from the DC side to the AC side. Each of the single-phase inverters can be a non-isolated single-phase inverter having a line connector and a neutral connector coupled to an isolated transformer winding, and the output windings of the transformer can be wired in a Wye configuration. Each of the single-phase inverters can include a local controller.
A bi-directional poly-phase DC/AC inverter is disclosed that includes an AC side and a DC side. The AC side receives and/or sources AC power, where the AC power includes a plurality of AC phases, each AC phase including a current waveform having a current amplitude and a current phase angle and a voltage waveform having a voltage amplitude and a voltage phase angle. The DC side receives and/or sources DC power. For each individual AC phase of the plurality of AC phases, the current amplitude of the current waveform is controlled independently of the current amplitude of the current waveform in the other AC phases, and the difference between the current and voltage phase angles is controlled independently the difference between the current and voltage phase angles in the other AC phases. The AC power can have three AC phases. The bi-directional poly-phase DC/AC inverter can include three separate bi-directional single-phase DC/AC inverters, each of the three separate single-phase inverters controlling a different one of the three AC phases.
Each of the single-phase inverters can be galvanically isolated from the DC side to the AC side. Each of the single-phase inverters can be a non-isolated single-phase inverter having a line connector and a neutral connector coupled to an isolated transformer winding; the output windings of the transformer being wired in a Wye configuration. Each of the single-phase inverters can be an H-bridge that includes of a plurality of power switching devices, and the power switching devices can be in parallel with diodes wired in an anti-parallel configuration. The power switching devices can be MOSFET transistors or IGBT transistors.